Methods and apparatus are known which provide a coating onto the outer surface of a compression molded article within the molding tool chamber during the molding process. Such coatings, commonly referred to as "in-mold coatings", provide a cosmetically improved surface layer on freshly molded articles where the molded surface otherwise would be aesthetically unacceptable due to molding blemishes caused by porosity, sinks or cracks and the like. Such blemishes may result from uneven plastic flow or shrinkage during cooling. Automotive vehicle body exterior panels are exemplary of compression molded parts requiring a surface quality improved over that provided by compression molding alone.
The in-mold coating process typically comprises first molding a base portion of the article in the cavity of a pair of relatively moveable die members of a die molding set, the base portion of the article is allowed to at least partially cure, after which a skin-forming coating material, i.e., the in-mold coating material, is injected onto that surface of such base portion which is to be improved. The base portion can be mechanically or hydraulically held in place against one of the die members and the second die member allowed to separate to provide a partial opening of the mold either prior to or simultaneously with injection of the in-mold coating material. Pressure is then re-applied to close the die members so as to distribute the surface forming in-mold coating material substantially uniformly over the surface of the base portion and thereby to substantially fill any porosity, sinks, cracks and like imperfections. Exemplary of the above, U.S. Pat. No. 4,082,486 discloses apparatus for providing an in-mold coating to a compression molded article during the molding process.
Injection devices are known for injection of the in-mold coating material into the molding cavity. Typically, the die members are opened slightly for injection of the coating material. Alternatively, the coating material can be injected into a precharge chamber which is opened within one die member and which is in communication with the molding cavity. Subsequently the coating material is displaced under pressure into the molding cavity without opening the die members except to the extent necessary to accommodate the added volume of the coating material. This later alternative is illustrated in aforementioned U.S. Pat. No. 4,082,486. In either case, a suitable injector device is required to deliver the liquid thermosetting resin coating material from a remote supply source. One suitable injector device, shown in aforementioned U.S. Pat. No. 4,082,486, is mounted on one die member and comprises a cylindrical housing. A feeder tube which is mounted within the housing has a cylindrical passage that extends axially from a supply port, which is adapted to be connected to a source of coating material, to a nozzle at the opening into the die member. A static mixer is provided within the cylindrical passage to mix the two components of the in-mold coating material. A conically formed head at the end of the feeder tube is moved forward to engage and close-off the nozzle opening into the die member. In such forward position, the end of the feeder tube is in sealing connection with the nozzle. The feeder tube is retracted away from the nozzle to open same and permit coating material to flow into the molding cavity (or precharge chamber).
The material employed for in-mold coatings typically comprises liquid thermosetting resin systems. These thermosetting resin systems will react and thus harden within the injector device if not quickly replaced by fresh resin or purged by solvent, for example methylene chloride, or by other means. During periods of active processing, the resin in the feeder channel is constantly replaced by fresh coating material. Prior to periods of inactivity, however, the coating material must be purged from the injector device to avoid clogging and damaging the injector device. The injector device typically is bolted to a die member and is not easily dismantled. Accordingly, to purge the injector device, solvent flush is fed into the injector device to flow through the injector nozzel opening into the molding cavity. It is necessary to collect the solvent flush as it exits into the molding cavity. This procedure is difficult and impractical and typically results in considerable splashing of resin-laden solvent into the molding cavity, from where it then must be cleaned, and also onto the operator and the surrounding areas.
The prior art of valve design teaches injection valves and related devices suitable for a variety of operations, none of which, however, meets the objectives of the present invention. U.S. Pat. No. 3,985,300 describes a self-purging injection valve adapted to inject additives through a central injection barrel into apparatus used in polymerization systems, for example into an extruder barrel or a polymerization vessel. A supply port feeds each of two channels through the central injection barrel. The channels are concentric, one being exterior and one interior a valve stem which is slideably mounted within the injection barrel. In a first position, the valve stem permits flow in the same direction through each of the two channels through an injection port into the polymerization system. In a second position the valve stem sealingly engages and so closes the injection port. This permits flow of purging fluid into and through one channel and back out the second channel to a purging waste port. The purging waste port can be closed during normal operation. The material supply channels communicate with each other at two locations, a first near where the valve stem forms a sealing engagement with the injection port, and a second at the supply port at the opposite end of the valve stem near the by means of the valve stem. Such design is ill-suited for use in in-mold coating application in view of the temperature-dependent, thermoset nature of in-mold coating material since, for example, no cooling means is provided. It appears unlikely that adequate cooling means could be provided to cool the supply channel which extends axially through the interior of the valve stem.
Other valves of various functions are known which likewise fail to meet the objectives of the present invention. In U.S. Pat. No. 2,283,762 a paint spray device is taught which provides two parallel tubes, a first for paint and a second for pressurized air. The spray nozzle, but not the paint supply tube, can be purged using the pressurized air. Purged material exits at the nozzle. U.S. Pat. No. 3,733,156 teaches a valve adapted to provide flow of material in alternating fashion from one only of a pair of screw-injection barrels. U.S. Pat. No. 3,752,298 provides a mixing head comprising a mixing chamber, a spray nozzle and a valve adapted to close a pair of lines feeding reactive additive plastics into the mixing chamber and to open separate lines to admit cleaning liquid. The material supply lines are not purged. U.S. Pat. No. 3,947,117 teaches a valve adapted to injecting into a mold a strand of a first plastic of ring-shaped cross section enclosing a strand of a second plastic. U.S. Pat. No. 4,260,348 teaches a water-cooled spruce bushing for an injection molding machine, which bushing has a central passageway for the flow of plastic from the nozzle of the machine to the mold.
It is an object of the present invention to provide an injection device for intermittently feeding in-mold coating material to a compression molding tool, which device can be mounted to a die member. It is a particular object of the invention to eliminate the need for purging coating material into the molding cavity. Specifically, it is an object to provide such a device which can routinely be totally purged of in-mold coating material to a remotely located collection means.
These and other objects and advantages of the present invention will be apparent from the following detailed description and drawings.